Arabidopsis Root Development is Regulated by the Endogenous Folate Precursor, Para-Aminobenzoic Acid, Through Modulation of the Root Cell Cycle

The continuous growth of roots depends on their ability to maintain a balanced ratio between cell production and cell differentiation at the tip. This process was reported to be regulated by the hormonal balance of cytokinin and auxin. However, other important regulators, such as plant folates, have also been reported to play a regulatory role. In this study, we investigated the impact of the folate precursor PABA on root development. Using pharmacological, genetic, and imaging approaches, we show that the growth of Arabidopsis thaliana roots is repressed by either supplementing the growth medium with PABA or overexpressing the PABA synthesis gene GAT-ADCS. This is associated with a smaller root meristem size, which consists of fewer cells. Conversely, reducing the levels of free root endogenous PABA results in longer roots with extended meristems and increased cell numbers. We provide evidence that PABA represses Arabidopsis root growth in a folate-independent manner and likely acts through two mechanisms: (i) the G2/M transition of cell division in the root apical meristem and (ii) promoting premature cell differentiation in the transition zone. These data collectively suggest that PABA plays a role in Arabidopsis root growth at the point where cell division and cell differentiation intersect.